Abstract
This paper describes a high-performance visual threshold detection system for FeIII at the ppb level. The technique is based on rapid changes of the mole fraction of ML- and M2L-type complex species about a ligand in a narrow range of FeIII concentration in a small excess, in case the second metal ion is bound to methylthymol blue (MTB), which can bind two metal ions at both ends of a π-electron conjugated system. The boundary of the color region in the FeIII concentration is readily controlled by the concentration of MTB. In this study, the visual threshold detection system was combined with the concentration technique using finely ground anion-exchange resin particles to improve the sensitivity. The complex species in a slightly acid solution were adsorbed quantitatively to the finely ground anion-exchanger. Filtration of the ion-exchanger using a membrane filter made of cellulose acetate brought about a superior color property with bathochromic shifts, as well as an enrichment effect. A color transition between the complementary colors through colorlessness, which was caused by the mixing of metanyl yellow as an inert dye, engendered successful performance of the visual threshold detection of FeIII at the ppb level.
